This invention relates to the measurement of lateral tracking error in turntable equipment for use in phonograph record disc reproduction. In particular this invention concerns a method and apparatus for obtaining a measure of or relating to such lateral tracking error at one or more predetermined radii.
When a master phonograph record disc is being cut, the cutting stylus or head traverses the rotating disc in a true radial direction. For accurate reproduction of record discs produced from the master it is therefore desirable that the pick-up stylus likewise traverses the rotating record disc in a true radial direction so that the stylus is continuously directed tangentially to the record groove, i.e. at all radii from the rotation axis. However, turntable equipment which achieves this is very complicated and expensive (e.g. Revox and B & O Series 4000). Accordingly the most common form of turntable equipment has the pick-up stylus mounted in a cartridge that is carried, e.g. via a so-called headshell, by an arm (often called a tonearm) pivotally mounted such as to move the stylus along an arc that only approximates to the true radial direction. To optimise the approximation and reduce the lateral tracking error, i.e. the degrees of arc away from the desired tangential direction, some turntable equipment has in the past included a very long tonearm which moved pivotally through only a small angle. Such equipment created attendant problems such as high mass and accordingly the present-day approach generally adopted is to provide turntable equipment as illustrated schematically in plan view in FIG. 1 of the accompanying drawings, this equipment having the cartridge 10 positioned at a predetermined angle .alpha. with respect to the tonearm 11 (the so-called offset angle) and arrange for the stylus 12 to "overhang" the centre spindle 13 of the turntable platter 14 by a predetermined distance "d". This overhang distance is the difference in length between the effective length "L" of the tonearm (as measured between the tonearm pivot 15 and the point or tip of the stylus 12) and the distance "P" between the tonearm pivot 15 and the axis of the turntable spindle 13 about which the platter 14 rotates, i.e. L=P+d.
Equations for the optimum relationships for "d", "L" and ".alpha." have been determined by H. G. Baerwald (1941, December "Jnl. Soc. Motion Picture Engineers" page 591), the optimum values depending on the minimum and maximum radii to be encountered. The generally accepted maximum radius is 146 mm for a 12-inch phonographic record disc and although a minimum radius of 56 mm can be encountered, the Internationally agreed IEC minimum radius is in fact 60 mm.
A difference in "d" of a few millimeters (e.g. 1 or 2 mm) can have a great effect on lateral tracking error. With appropriate settings for the offset angle .alpha. and overhang distance "d" on any particular turntable equipment, it can be arranged that zero tracking error is obtained at two discrete radial distances from the platter or spindle axis. For optimum results across the whole of a 12-inch phonographic record disc it has been calculated that these two radial distances should be at 66 mm and 121 mm radii where the minimum and maximum radii are 60 mm and 146 mm, or at 63.5 mm and 119.5 mm where the minimum and maximum radii are 58 mm and 146 mm, or at 61.5 mm and 118.5 mm where the minimum and maximum radii are 56 mm and 146 mm. For the sake of simplicity and without intending any specific limitation to the scope of this invention, reference will hereinafter be made solely to the case where the minimum and maximum radii are such that optimum zero tracking error occurs at 66 mm and 121 mm radii. To achieve this optimum result, recourse is had to a so-called alignment protractor which comprises a piece of card pierced to fit over the spindle 13 and overlie the platter 14, the card being marked with a single radial line and a plurality of equi-spaced parallel transverse lines intersecting the single radial line at right angles, the radial points at 66 mm and 121 mm being marked on the single radial line. In use, the arm and card are both pivoted about their respective axes and the cartridge position adjusted until the cartridge sides and the transverse lines are parallel to one another (whereby the stylus is directed parallel to the transverse lines) and the point of the stylus coincides with one of said marked radial points. The arm and card are again both pivoted to repeat the procedure for the other marked radial point. It will be appreciated that this method, which primarily involves adjusting the overhang, is somewhat complicated (stylus point engagement being difficult to view) and only achieves zero tracking error at two points, providing no information as to the value of tracking error occurring at other radial locations. Furthermore, in the case of some turntable equipment the subject of faulty design and/or a failure to give practical effect to the Baerwald equations, even such optimum results cannot be achieved, i.e. zero tracking error cannot be obtained at two points at all.